1. Field of the Invention
The present invention relates to a methanol production process starting from hydrocarbon, and more particularly to an improved methanol production process which lessens generation of waste water and saves the amount of boiler feed water.
2. Description of the Related Art
For example, Japanese Patent Provisional Publication No. 1-180841/1989 discloses that a process for producing methanol from hydrocarbon, generally consists of the following steps:
(1) a synthesis gas preparation step wherein steam is reacted with gaseous hydrocarbon or vaporized hydrocarbon liquid on a nickel-based catalyst at a temperature of 800 to 1000.degree. C. in a reforming furnace, thereby preparing synthetic gas containing hydrogen, carbon monoxide and carbon dioxide as main components; PA1 (2) a synthesizing step wherein the synthesis gas is reacted on a copper-based catalyst at a pressure of 50 to 150 atm and a temperature of 200 to 300.degree. C., and the resulting crude liquid methanol is separated from the reacted gas; and PA1 (3) a distilling step wherein the crude liquid methanol is distilled through one or more distillation columns, thereby separating refined methanol from waste water containing one or more organic compounds having lower boiling points than that of methanol (hereinafter referred to as lower b.p. organic compounds), and one or more organic acids as well as one or more organic compounds having higher boiling points than that of the methanol (hereinafter referred to as higher b.p. organic compounds). PA1 (a) a large number of higher b.p. organic compounds. For example, higher alcohol having a carbon number of 2 or more and paraffins having a carbon number of 14 to 60. PA1 (b) organic acid salt and ester such as those of formic acid. PA1 (c) alkali metal salt. PA1 (1) a process for producing methanol from hydrocarbon comprising the steps of (a) reacting hydrocarbon with steam to generate synthesis gas containing hydrogen, carbon monoxide and carbon dioxide as main components, (b) reacting the synthesis gas on a catalyst for methanol synthesis and recovering crude liquid methanol from the reacted gas, and (c) distilling the crude methanol into refined methanol and waste water containing lower b.p. organic compounds, higher b.p. organic compounds and organic acids, wherein the hydrocarbon comes in contact with the waste water which has been neutralized with alkali metal salt or hydroxide in the step (c) so as to be humidified, and comes in contact with condensed water separated from the synthesis gas obtained in the step (a) so as to be further humidified, and is then supplied to the step (a); and PA1 (2) As one of the preferred embodiments for (1), the process for preparing methanol synthesis gas from hydrocarbon, wherein when the hydrocarbon comes in contact with the waste water which has been neutralized with the alkali metal salt or hydroxide at the step (c) so as to be humidified, the hydrocarbon preheated to a temperature of 250 to 430.degree. C. adiabatically comes in contact with the waste water so as to be humidified.
In the synthesis gas preparation step, usually, it is necessary to supply steam having the three times moles of the number of carbons of hydrocarbon used as a starting material. For example, 3 m.sup.3 of H.sub.2 O is added for 1 m.sup.3 of CH.sub.4. Thus, a large amount of steam is required for preparing methanol synthesis gas. By utilizing heat generated at each preparation step, steam is recovered from water having a high purity so as to be used for a process.
In the synthesizing step, methanol is produced from carbon monoxide and hydrogen in the synthesis gas, while methanol and water are generated from carbon dioxide and hydrogen. The water and some impurities are contained, together with the methanol, in the crude liquid methanol, and are separated at the next distillation step. The separated water is discharged from the system without being utilized further.
Japanese Patent Provisional Publications Nos. 51-115505/1976, 55-139492/1980, 60-245997/1985, 57-18640/1982, and 1-180841/1989 disclose process for reducing the amount of steam required for the conventional synthesis gas preparation process.
In the methanol production process described above, a large amount of expensive water with high quality is needed as boiler water. Since the water separated in the distillation step contains the following components and can be hardly utilized, it is discarded as it is.
The organic acid salt and ester have high acidity and corrosivity. For this reason, for example, alkali metal hydroxide such as sodium hydroxide, or carbonate such as sodium carbonate is added for neutralization. Consequently, alkali metal salt is contained in waste water in the distillation step as described in the Japanese Patent Provisional Publication No. 57-18640/1982.
It is extremely difficult to remove these impurities from the waste water obtained in the distillation step, and a great deal of cost is required to utilize the waste water as boiler water for a methanol production process. Therefore, the waste water is discarded without being further utilized.
Furthermore, since the waste water contains the said components, it is hazardous to the public and needs to be subjected to a waste water treatment such as a biological treatment.
Thus, the waste water obtained in the distillation step contains a large number of organic compounds and the like and should be subjected to the waste water treatment. For this reason, a great deal of cost is required. If the waste water can be utilized for the process, the required amount of the expensive boiler water can be reduced. Therefore, it has been desired that the waste water should be recycled.
The present inventors disclose a process for process steam reduction, being performed at a process in which an alkali metal compound is not added in the distillation step as described in the Japanese Patent Provisional Publication No. 1-180841/1989. In this process, stainless materials having nickel and chromium are placed for materials of a distillation column and a hydrocarbon humidifier. The amount of waste water to be supplied is set to 1/10 to 1/20 as much as that for a conventional process in order to prevent the concentration of organic acid from increasing in the humidifier. Thus, corrosion of the material caused by the organic acid can be avoided and the waste water can be utilized.
According to the above-mentioned process, while the amount of the waste water can be decreased and the boiler water can be reduced, cost is increased because stainless steel having a higher grade than carbon steel is used for materials of the humidifier and the distillation column.